When air is drawn into an air compressor, all the constituent gases and vapors in the ambient atmosphere are drawn into the compressor in the same proportion as the proportions in the ambient atmosphere. For example, if a compressed air system provides air at a gage pressure of 102.9 psi, and the ambient atmosphere has an absolute pressure of 14.7 psi, the absolute pressure has increased by a factor of eight. Vapors in the atmosphere, notably water a vapor, which may be at a partial pressure below the dew point at the temperature of the ambient atmosphere, are compressed to a partial pressure which is increased eightfold, and may greatly exceed the dewpoint for air at the temperatures in the compressed air system. Hence, the vapors, notably water vapor, condense in the compressed air system, causing water to collect at low points in the system.
Water has a density vastly greater than does air, and hence acts very differently from air when it flows through valves, flow conduits, and orifices of a compressed air system. Indeed, if a mixture of water and air flow through an orifice or valve, the air may drive masses of water at a high speed, which act like bullets, causing noise and damaging the system. Also, if an air cylinder used for cushioning a force becomes filled with water, it becomes rock hard and does not provide the required cushioning.
Hence, compressed air systems generally have water release valves which are opened occasionally to eject water, or a mixture of water and air. An example of a prior art valve is given in U.S. Pat. No. 4,928,724 entitled "Automatic Wet Tank Drain Valve". This valve is activated by electrical or mechanical means. Other drain valves are operated manually.
Automatic valves of the prior art are generally not very satisfactory. For example, some of them, after being opened, permit a major loss of pressure from the compressed air system before closing. For example, such valves may lose as much as 20% of their pressure after a valve is opened, and before it closes.